Happy Birthday, Pluto! (You’re Still Not a Planet, Though)

On February 18, 1930 a young astronomer named Clyde Tombaugh discovered a new object in the outer Solar System by examining photographic plates he had taken several weeks earlier. After a lot of discussion and suggestions, the name Pluto was selected for the new object, which was elevated to the status of the 9th planet in the Solar System.

Since 1930, we’ve learned a lot more about Pluto–and also about our Solar System. From the initial mass estimate ranking Pluto about the same as Earth, scientists have used the motion of Pluto’s largest satellite Charon to measure its mass to be about 18% of the mass of our Moon. (Mercury, by comparison, is about 5 times more massive than the Moon.) In 2003, another outer Solar System object later named Eris was discovered to be roughly the same size as Pluto, but slightly more massive. Along with the discovery of other distant objects with great names like Quaoar, Haumea, Makemake, Orcus, and Sedna, it became evident that Pluto isn’t particularly unique. In 2006, the International Astronomical Union (IAU) drafted a formal definition of what a “planet” is, which excluded Pluto (along with Eris and the other new discoveries).

A lot of people I’ve talked to seem to take it rather personally that Pluto has been “demoted”. As a cosmologist I don’t have a stake in this discussion (nobody consulted me, even! I’m offended!)—I don’t study planets or the Solar System, and as odd as it might seem, I don’t particularly care if Pluto is classified as a planet or not. But one thing is fairly certain: if Pluto is classified as a planet, there will be a lot more planets in the Solar System than the 9 we memorized in elementary school. No matter how you classify things, 9 is a number that doesn’t work: there are either 8 planets (under the current IAU definition) or a lot more. Keeping Pluto as a planet and excluding the others can only be justified by nostalgia, not science.

However, for the record I think we should teach elementary school kids how to spell “Quaoar”.

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